Many fuel cell systems use fuel cells in which a fuel electrode and an oxidant electrode are placed on respective sides of an electrolytic membrane, power is generated by an electrochemical reaction between hydrogen supplied to the fuel electrode and oxygen in the air supplied to the oxidant electrode, and water is produced at the oxidant electrode.
When such a fuel cell is operated at a temperature which is lower than a normal operation temperature, a predetermined voltage and a predetermined current cannot be output, and, thus in many cases a warm-up operation is executed after startup, until the normal operation temperature is reached. As a method of the warm-up operation, there is proposed a low-efficiency operation method in which the amount of supply of air supplied to the fuel cell is set lower than the normal amount of supply, and an air stoichiometric ratio is set lower than the air stoichiometric ratio used during normal operation (for example, refer to Patent Literature 1). The air stoichiometric ratio refers to a ratio of the actual amount of air with respect to a theoretically necessary amount of air corresponding to the load of the fuel cell, and the air stoichiometric ratio during normal operation is usually about 2.0. When the air stoichiometric ratio is set to be low and the low-efficiency operation is executed, the air concentration overvoltage becomes larger than that during normal operation, and, thus, of the energy that can be extracted from the reaction of hydrogen and oxygen, the thermal loss (power generation loss) is increased.
In addition, there is also proposed a method in which, when the fuel cell is started up under a low temperature environment, in order to stably execute the warm-up operation of the fuel cell and inhibit degradation due to insufficient supply of gas, the air stoichiometric ratio is changed between 2.0, which is a standard value, and 1.8; a hydrogen stoichiometric ratio is changed between 1.2, which is a standard value, and 1.05; a cell stoichiometric ratio of each unit cell is calculated based on a percentage of change of the cell voltage during this process; and, when the stoichiometric ratio of each unit cell is lower than a predetermined value, the amounts of supply of air and hydrogen are increased (for example, refer to Patent Literature 2).